A low-power transponder device for use in identification systems is disclosed in IEEE Journal of Solid-State Circuits, Vol. 30, No. 3, March 1995, pages 306-310, “A Low-Power Transponder IC for High Performance Identification Systems” by Ulrich Kaiser and Wolfgang Steinhagen. In such a transponder device, the LC oscillator circuit is used as an RF interface to receive energy from a reader unit and to send data back to the reader unit. The reading unit sends signal bursts at the resonant frequency of the transponder's LC oscillator circuit. In the transponder device, the RF signal inductively received through the LC oscillator circuit is rectified and fed to a storage capacitor. Reception of RF energy is recognized in the device with an end-of-burst (EOB) detector. The output of the EOB detector determines the mode of operation of the transponder device. When RF energy is received, the device is operated in a charge mode and energy is stored in the storage capacitor CL. When the transmission of RF energy ends, this is detected by the EOB detector, and the device is switched to a transmit mode. In the transmit mode, oscillation of the LC oscillator circuit is sustained by applying stimulating pulses to the circuit with an appropriate timing, intensity and phase. The stimulating pulses are also referred to as “plucking pulses”. Frequency shift keying (FSK) is typically used to transmit data from the transponder device to the reader unit. To change the frequency of the oscillator signal, a small capacitor is selectively connected across the terminals of the LC circuit. In such a transponder device, the energy needed to transmit data from the device to the reader unit is entirely taken from the storage capacitor so that the device may be battery-less.
The stimulating pulses are provided by a pulse generator in response to the output from a peak detector connected to the LC oscillator circuit. Since the LC circuit usually has one terminal connected to the storing capacitor and another terminal connected to ground through a rectifier diode, the peak detector should detect the minimum peak voltage of the oscillator signal. A switching transistor momentarily connects the node between the LC circuit and the rectifier diode to ground in response to each stimulating (or plucking) pulse received from the pulse generator.
Depending on the Q-factor of the LC circuit, the stimulating pulses are needed more or less frequently. For example, at a Q of 100, a pulse is needed every 8th period of the oscillator signal; at a Q of 50, a pulse may be needed every 4th period of the oscillator signal.
At Q-factors as low as 10 to 30, the stimulating pulses must be provided even more frequently. It has now been found that over-stimulation may occur under these circumstances. As used here, “over-stimulation” means that the negative peak voltage of the oscillator signal exceeds a level at which the EOB detector reacts, thereby attempting to switch the device from the transmit mode to the charge mode. This causes corruption of the data that are in the process of being transmitted from the device to the reader unit.